1. Technical Field of the Invention
The present invention relates to mobile communications, in particular, to a system and method for improving the position estimates for locating a mobile station within a mobile telecommunications system, and, more particularly, to a system and method for resolution of mobile position estimates from as few as two base stations.
2. Description of Related Art
One feature of the emerging field of digital wireless telecommunications receiving increasing development, e.g., Time Division Multiple Access (TDMA), is that of mobile positioning. Clearly, an accurate mobile positioning system would be an attractive feature to subscribers and, undoubtedly, would present cellular telephone providers with additional revenues. However, recent development efforts in mobile positioning are not simply the result of potential commercial gains. Recent legislation requires the implementation of emergency location services, e.g., in some digital wireless systems in the near future, which is most likely the primary catalyst for the recent surge in mobile positioning research and development.
TDMA mobile communications systems can be either inter-cell synchronous or inter-cell asynchronous systems. In other words, the base transceiver stations (BTSs) in an inter-cell synchronous system are accurately synchronized with one another, and the BTSs in an inter-cell asynchronous system are not. More specifically, asynchronous BTSs do not share a common time reference, and their transmissions, therefore, have arbitrary timing relative to each other. An example of an inter-cell synchronous system is the North American IS-95 system. Examples of inter-cell asynchronous systems include the Wideband Code Division Multiple Access (WCDMA) systems proposed in the CODIT, ETSI SMG2 Group Alpha, and ARIB technical specifications and the Global System for Mobile Communications (GSM).
A number of disadvantages exist with inter-cell synchronous systems. One prerequisite for such systems is a high level of synchronization among the various BTSs, within the synchronous system, with the degree of synchronization generally measured in microseconds (xcexcs).
However, an area in which the synchronous network has shown particular advantage over non-synchronous networks is in mobile positioning applications. Synchronous networks have previously shown a distinct advantage over non-synchronous networks since, by design, the synchronous networks share a reference clock. In particular, when an MS is within communication range of three or more BTSs, the MS can tune or transmit, e.g., an access burst, to the BTSs substantially simultaneously. Time delay of arrival measurements, for example, may then be made by each of the BTSs, and respective MS to BTS distances are then made therefrom. With at least three such measurements in such conventional systems, an accurate position is then easily made by use of triangulation, as is understood in the art.
When BTSs are operating asynchronously, however, the task of location calculation is complicated by the fact that each BTS is operating on clocks independent from one another. Therefore, while one BTS is receiving data on Time slot Number 1 (TN1), for example, a neighboring BTS might be simultaneously receiving on TN3. To make an effective location determination, the respective delay of MS transmission to BTS reception must be ascertained along with the relative TN offset with respect to the other BTSs participating in the location calculations. However, recent advances in asynchronous networks have effectively overcome this disadvantage, resulting in an asynchronous network having an equivalently attractive infrastructure for implementing mobile positioning in the aforedescribed synchronous network.
Mobile stations are, in general, within reception range of a number of BTSs, where a traffic channel is maintained between the mobile station and that BTS exhibiting the best communication characteristics, e.g., signal to interference ratio. As is understood in the art, however, when radio frequency (RF) characteristics decline below a specified level or when RF characteristics from another BTS increase beyond a specific threshold relative to the current BTS with which the mobile station is maintaining the traffic channel, a handover is initiated where another traffic channel is setup between the mobile station and the BTS exhibiting the better communication characteristics and, substantially concurrently, the traffic channel between the mobile station and BTS previously in use is broken.
It is this situation of the MS being within range of a number of BTSs that is exploited in the majority of current positioning techniques. For example, one well known network-based method for determining the position of cellular mobile station is disclosed in commonly-assigned Swedish Patent Application No. 9303561-3 to R. Bodin. In order to determine the position of a mobile station, a handover procedure is initiated between a serving base station and the mobile station. The mobile station transmits access request signals to a new base station. The BTS measures the time delay for the access request signal to travel between the mobile station and the base station. This procedure is repeated between the mobile station and one or more additional base stations. A service node in the cellular network calculates the position of the mobile station by utilizing information about the known positions of the base stations and the measured access time delays.
This network-based method of determining the position of cellular mobile stations relies on asynchronous handovers, where the target base station measures the access delays to the mobile station. Each access delay is used as a measure of the distance between the mobile station and the respective base station. At least two positioning handover operations are therefore needed to obtain three such distances, which can be used in a triangulation algorithm to determine the mobile terminal""s position. Notably, one distance can be obtained between the serving base station and the mobile terminal without a positioning handover. For example, in GSM, the Timing Advance (TA) value used for time alignment of bursts can optionally be used as a representation of the distance in the serving cell. A more accurate position determination can be attained if more than two such positioning handovers are made, because more than three distances will be known. The use of more than three distance measurements compensates for some errors arising in the individual measurements.
The positioning handovers are made in sequence, with each handover, e.g., in a GSM network taking approximately 0.5 seconds to complete. In fact, the mobile station""s timeout procedure takes about 0.3 seconds alone to complete. Consequently, a shortcoming of the above-described sequential method is that the total time it takes to determine the mobile station""s position is proportional to the number of cells in which the positioning handovers are made. However, a more significant disadvantage of this and other similar prior art systems is the fact that no provisions are made for a relatively common positioning scenario when the MS is within range of only two BTSs.
As is understood in the art, if the MS is in communication range with only two BTSs, an ambiguity of position will exist as the position will, at best, be confined to two distinct geographical locations. With reference now to FIG. 1, there is illustrated a scenario where the MS is only within the communications range of BTSs 110 and 120, respectively. Each of BTSs 110 and 120 employ a three-sectored cell using antennas with pointing azimuths of 120 degrees, as is well understood in the art. Cells 125, 130 and 135 are thereby covered by BTS 110, and cells 140, 145 and 150 are covered by BTS 120. Timing advance measurements from BTSs 110 and 120, represented by the circular distance traces 110A and 120A, respectively, intersect at two points, indicated as A and B in FIG. 1, representing the two possible locations of the MS therebetween. As noted in the figure, point A is within cell 140 and point B, the other positional option, is located within cell 135, located not only at a remote geographical position but within a different cell under the control of a different BTS, and possibly a different Public Land Mobile Network (PLMN) system.
Current time-based positional algorithms and techniques are for the most part unable to accurately determine the position of a mobile station using only two BTSs (unless prohibitively expensive array antennas are employed) and cannot resolve the positional ambiguity illustrated in FIG. 1. Accordingly, only by using three or more BTSs, i.e., by triangulating, are present time-based techniques able to pinpoint the position of a mobile station and resolve the aforedescribed positional ambiguities inherent in a two-BTS measurement, as illustrated and described in more detail in connection with FIG. 3.
Obviously, however, an MS will often be positioned such that communication with three BTSs is unachievable, e.g., due to the geographical distances between the MS and BTSs, a local BTS being inoperative, etc. It is, therefore, desirable that a positioning system have the capability to resolve an MS positional ambiguity using only two BTSs.
It is, accordingly, a first object of the present invention to provide an improved system and method for measuring the position of a mobile station (MS) within a digital mobile telecommunications network.
It is also an object of the present invention to enable measurement of an MS position within the digital mobile telecommunication network when the MS is within the communications range of as few as two BTSs.
It is a further object of the invention to enable measurement of an MS position when the MS is within communications range of only two BTSs operating synchronously or asynchronously.
In accordance with the present invention, a system and method are provided for determining the location or position of a mobile station (MS) within a digital wireless network when only two base stations are available for time of arrival or other triangulation measurements. A call setup is initiated concurrently with the gathering of timing advance or other such location data on the targeted MS, thereby allowing cell sector information to be collected. This data, along with the locating data, is then reported to a Master Positioning Center where distance calculations are made from the locating data. Positional ambiguities are then resolved with a comparison to the serving cell identity recovered during the call placement to the mobile when the locating data was being taken.